This invention relates to level indicators, and particularly relates to external level indicators for determining the level of the interface between a mass of flowable material and the void volume above the mass of flowable material within a container.
Liquid level measuring devices have been known for many years. Their purpose is to locate the level of a flowable material, or to indicate the amount of flowable material remaining in a container.
On many occasions, monitoring the amount of flowable material in a container is required. However, direct observation of the flowable material level is not always possible or practical. Measurement of the material in such containers as pressurized cylinders, sealed containers, cryogenic flasks, and opaque vessels is often difficult. Such measurements are even more troublesome when the material within the container is corrosive or potentially toxic or flammable.
Sight glasses and weight scales are some examples of liquid level measuring devices which are commonly employed. Both of these devices suffer from a number of disadvantages. Sight glasses are expensive, and they can crack and break easily. On such occasions where the container is placed outdoors, ultraviolet light can cause the glass to haze. Weight scales are also expensive, and in many instances, measurements provided by weight scales are inexact.
A simple, economical external liquid level gauge which permits a direct reading of the level of a flowable material has been provided by the present inventor in Canadian Pat. No. 1,177,281 issued on Nov. 6, 1984. The liquid level measuring device taught therein employs one thermochromatic material which is coated onto a base layer. The base layer is magnetically mounted to the outside surface of the outside wall of the container, and thus the external liquid level gauge can be repeatedly removed and replaced or relocated when necessary.
The theory is that the rate of heat transfer is different between a mass of flowable material and the void volume above it such that for any container with a modest heat conducting capability, the container wall experiences a temperature gradient which is most pronounced at the interface of the contents with the void volume above the contents, and of course below that interface. That is to say, the rate of heat transfer through the wall of a container will be greater where there is a mass of flowable material located in the container than where there is a void volume above the flowable material. In other words, the rate of heat transfer through the container wall changes most abruptly at the level of the interface, and below. Thus, with the use of a thermochromatic material, a vivid color change occurring at the interface, and below, will permit an observer to obtain a direct reading of the level of the flowable material within a container by discerning where the interface is located.
In a co-pending U.S. patent application, Ser. No. 10/077,971 filed Feb. 20, 2002, for xe2x80x9cExternal Liquid Level Gauge,xe2x80x9d the present inventor teaches an external liquid level gauge which is adapted to be affixed vertically to the outside wall of a container. This co-pending U.S. Patent Application claims priority from Canadian Patent Application No. 2,338,407 filed Feb. 26, 2001. The external liquid level gauge as taught therein is in the form of an elongated strip and it comprises a layer of base material and a layer of thermochromatic materials. Furthermore, the thermochromatic layer comprises a light absorbing background and at least two regions of thermochromatic materials which are arranged upon the light absorbing background. The regions of at least two thermochromatic materials are disposed in arrays thereof and are arranged entirely along the length of the external liquid level gauge. Moreover, each of the thermochromatic materials responds chromatically within a different operating temperature range.
Several other prior art thermochromatic external liquid level gauges are now described. They include GILMOUR U.S. Pat. No. 3,696,675 issued Oct. 10, 1972, which teaches an external liquid level gauge adapted to be permanently affixed to the outside wall of a container for determining the liquid-gas interface within the container. The external liquid level gauge described therein consists of a uniform thermochromatic liquid crystalline material which coats the entire base layer of the gauge such that it is at right angles to the liquid-gas interface. The uniform thermochromatic material covers the entire temperature range to which the container is subjected within an overall range of xe2x88x9220xc2x0 C. to 250xc2x0 C. Depending upon the thermochromatic material selected, color changes over a gradient from violet to red can occur in a range as small as 2xc2x0 C. to one as broad as 150xc2x0 C. Since the temperature differential across the liquid-gas interface is generally small, on the order of less than 2xc2x0 C., the change in color is slight across the interface. This is particularly the case when the container is placed outdoors and a large temperature range needs to be covered. As a result, it is difficult to visually locate the liquid-gas interface.
In U.S. Pat. No. 5,323,652 issued Jun. 28, 1994 to PARKER, the inventor teaches a thermochromatic level indicator for determining the level of a material inside a container. The thermochromatic level indicator includes at least two thermochromic materials of different opacities and transition temperature. Prior to the attachment of the thermochromatic level indicator to the outside surface of the outside wall of the container, the thermochromic materials are applied to a transparent film by silk screening, other printing and coating methods, or methods which employ the use of microencapsulated thermochromic materials. The thermochromatic level indicator may be permanently adhered to the container wall or it may be adhered to a magnetic strip which can be temporarily affixed to the container wall.
In another U.S. Pat. No. 5,707,590, issued Jan. 13, 1998, the inventor THOMAS et al. has provided a detergent container with a thermochromatic level indicator. In one embodiment of the invention, the thermochromatic substance is added to the container""s plastic material during the molding process. In another embodiment of the invention, the level indicator or strip comprises a base material, such as Mylar, which is coated or imbedded with a thermochromatic substance by such methods as painting, stripping, or screen printing.
The foregoing prior art level gauges are useful for indicating the amount of materials remaining inside a container, but they are not specifically useful as overfill indicators. Indeed, in many instances, only an approximation of the level of materials inside the container is provided. Since the thermochromatic materials present in the prior art level gauges are not in direct contact with the outside surface of the outside wall of the container, the chromatic response of these thermochromatic materials may be delayed. Furthermore, these prior art level gauges may not provide a prominent color change at the level of the interface, and thus a reading of the level of materials may be inexact.
However, under certain circumstances, it is critical to determine the precise level of the materials inside a container, such as in the case of liquefied propane in pressurized cylinder. In warm weather conditions particularly, hydrostatic pressure exerted by liquefied propane inside the pressurized cylinder may cause the cylinder to explode if the cylinder is overfilled. Thus, in order to prevent undesirable gas venting from overfilled cylinders, the United States National Fire Protection Association (NFPA) has recently mandated a safe-fill level of propane in pressurized cylinders to be at a level which is 80% of the volume of the propane cylinders such that a 20% volume head space is maintained when the pressurized cylinders are full of propane. Due to the new NFPA regulations, float valves are being applied to cylinders so as to prevent overfill. The float valve closes when the propane level reaches a volume of 80% of the container.
Although float valves provide a method of preventing overfilling, they are a costly solution. In order for a float valve to be installed in a cylinder, the structure of the cylinder needs to be altered. Indeed, some existing cylinders on the market may be retrofitted with float valves, many other existing cylinders must be discarded, and new cylinders with pre-installed float valves need to be manufactured in order to comply with the new regulation. Furthermore, since float valve is a mechanical device, it is subject to mechanical failure over a period of time.
As is discussed immediately above, installing float valves to cylinders is a solution to prevent propane overfill. However, these float valves may only be compatible to cylinders used in the United States and Canada, and may not be compatible with cylinders used overseas, in such places as Europe, Asia or South America where propane gas is a commonly used cooking fuel. Moreover, many regions outside of North America may not yet have the same or similar regulations as mandated by NFPA, but insurance companies, local municipalities, and the like, may require overfill indicators in order for certain coverage or licensing regulations to be effective.
In light of the foregoing, there is a need to provide a level indicator which can be adapted to any container for determining the level of the materials inside the container, and to detect overfill. Furthermore, there is a need to provide a level indicator which can be easily installed to a container without having to alter the structure of the container, and without the use of tools during the installation process. Still further, there is a need to provide a level indicator which can be provided to the user at a relatively low cost.
As employed herein, the term xe2x80x9cflowable materialxe2x80x9d is intended to mean any fluidic matter in which the shape of a given mass depends on the container but the volume is independent thereof. xe2x80x9cFlowable materialxe2x80x9d is also intended to mean any fluid matter which seeks a level and offers no permanent resistance to change of shape. The term may include any mass of granular material which has fluidic properties.
The expression xe2x80x9cthermochromatic materialsxe2x80x9d as used herein is intended to mean materials that have or exhibit different colors or shades of color at different temperatures. The expression xe2x80x9cresponding chromaticallyxe2x80x9d as used herein is intended to mean having or exhibiting different colors or shades of color at different temperatures.
It is an object of the present invention to provide a novel level indicator to determine the level of the interface between a mass of flowable material and the void volume above the mass of flowable material within a container, and which obviates or mitigates at least one of the disadvantages of the prior art.
In accordance with one aspect of the present invention, there is provided a level indicator which is intimately affixed to the outside surface of the outside wall of a container for use in determining the level of the interface between a mass of flowable material and the void volume above the mass of flowable material within the container.
The flowable material within the container has fluidic properties, and it has a faster rate of heat transfer than the void volume above it within the container. The flowable material is chosen from the group consisting of liquefied gas, corrosive liquid, water, alcohol, oil, coffee, tea, juice, milk, and granular material.
In keeping with the present invention, the container to which the level indicator is intimately affixed to the outside surface of the outside wall may be chosen from the group of containers consisting of pressurized cylinders, open containers, sealed containers, cryogenic flasks and opaque vessels.
The level indicator of the present invention is in the form of an elongated strip, vertically extending along the height of the container. In keeping with the present invention, the level indicator conforms to the contour of the outside surface of the outside wall of the container such that the level indicator is in intimate heat transfer relationship with the outside wall of the container.
The level indicator includes a thermochromatic layer which has at least two thermochromatic materials. The at least two thermochromatic materials are arranged in at least two regions. Furthermore, each of the at least two regions of thermochromatic materials is arranged in an individual area upon the outside surface of the outside wall of the container. The individual area may be chosen from the group of geometric configurations consisting of dots, circles, stars, squares, triangles, arrows, semi-circles, pentagons, hexagons, digits and letters. Each of the thermochromatic materials responds chromatically within a different operating temperature range. The regions of the at least two thermochromatic materials are disposed in arrays thereof arranged along the length of the level indicator.
In one embodiment of the present invention, the level indicator vertically extends along the height of the container, at least in the region of the top portion of the container, and at least above the legal safe-fill level of the flowable material within the container such that overfilling of the flowable material in the container is detectable.
The level indicator of the present invention may also include a layer of base material which is intimately affixed to the outside wall of the container, and is such that it is in intimate heat transfer relationship with the outside wall of the container. The thermochromatic layer overlies the layer of base material.
The layer of base material is applied to the outside wall of the container by an application method chosen from the group consisting of directly applying the base material to the outside surface of the outside wall of the container, and removably securing the base material to the outside surface of the outside wall of the container when the base material additionally has adhesive or magnetic properties so as to permit the level indicator to be repeatedly removed and reattached to the outside wall of the container.
When the layer of base material is directly applied to the outside wall of the container, the at least two thermochromatic materials are applied to the base material by an application method chosen from the group consisting of painting, silk screening, stencilling, molding, crayon transfer, pencil transfer, screening, and combinations thereof.
Typically, but not necessarily, the base material has a series of indicia printed thereon, and the indica are vertically positioned along the length of the base material and at predetermined spaced-apart locations. The indicia on the base material remain invisible until such time when at least a portion of one of the at least two thermochromatic materials of the thermochromatic layer becomes activated. When activated, that portion of one of the at least two thermochromatics becomes translucent, so as to reveal at least one of the indicia printed on the base material.
In one aspect of the present invention, the base material has light absorbing properties.
In another aspect of the invention, the thermochromatic layer further includes a light absorbing background upon which the at least two regions of thermochromatic materials are arranged.
In another embodiment of the invention, the at least two thermochromatic materials are directly applied to the outside surface of the outside wall of the container. Each of the at least two thermochromatic materials is directly applied to the outside surface of the outside wall of the container by an application method chosen from the group consisting of painting, silk screening, stencilling, molding, crayon transfer, pencil transfer, screening, and combinations thereof. Furthermore, each of the at least two thermochromatic materials is carried to the outside surface of the outside wall of the container by a transfer medium. In this particular embodiment, at least a portion of the outside surface of the outside wall of the container which underlies the level indicator has been treated by an application method chosen from the group consisting of anodizing, anti-oxidizing, rust-proofing, and combinations thereof, so as to provide at least that portion of the outside surface of the outside wall with a light absorbing property, prior to the direct application of the level indicator to the outside surface of the outside wall of the container.
Still further, the level indicator may include a protective lamina which overlies the at least two thermochromatic materials. Typically, but not necessarily, the protective clear lamina is an ultraviolet filter.
The thermochromatic materials are chosen from the group of cholesteric liquid crystal compounds and mercurous oxide. In keeping with the present invention, at least two thermochromatic materials in the array have overlapping temperature ranges.
The regions of thermochromatic materials are arranged on the outside surface of the outside wall of the container chosen from the group of arrays consisting of one of the regions of thermochromatic materials vertically positioned down the center of the level indicator and at least one other region diagonally positioned on each side of the vertically positioned region of the level indicator, all of the regions of thermochromatic materials vertically positioned upon the level indicator, and all of the regions of thermochromatic materials horizontally positioned upon the level indicator.
In a particular embodiment of the present invention, the at least two adjacent regions of thermochromatic materials are arranged to form a set, and the set is disposed vertically along the length of the level indicator in a repeated manner.
Another object of the present invention is to provide a method of determining the level of the interface between a mass of flowable material and the void volume above it within a container using a level indicator. The level indicator would be, of course, as described above. The method comprises the steps of:
(i) inducing heat transfer between the level indicator and the mass of flowable material within the container; and
(ii) discerning visually a color change in the at least one region of the array of the level indicator.
The region noted above which responds chromatically to a temperature change is contiguous to the mass of flowable material within the container. Specifically, step (i) may be achieved by any of the steps chosen from the group of steps consisting of:
(a) spraying a liquid onto the outer surface of the level indicator;
(b) wetting the outer surface of the level indicator, with a moistened cloth or sponge;
(c) pouring a liquid down the outer surface of the level indicator;
(d) trickling a liquid down the outer surface of the level indicator; and
(e) applying an electrically energized source along the length of the level indicator.
In one embodiment of the present invention, the liquid as employed above in any of steps (a) through (d) is a heat source. Since the temperature of the liquid is above the temperature of the flowable material within the container, heat transfer is induced from the liquid to the flowable material.
In another embodiment of the present invention, the liquid as employed above in any of steps (a) through (d) is a heat sink. Here, the temperature of the liquid is below the temperature of the flowable material within the container. Thus, heat transfer is induced to the liquid from the flowable material within the container.
In yet another embodiment of the present invention, at least two adjacent regions of the level indicator are arranged to form a set, which comprises the at least two thermochromatic materials. The set is disposed vertically along the length of the level indicator in a repeated manner.
Particularly when a plurality of sets are disposed in a repeated manner vertically along the length of the level indicator, the method of determining the level of the interface between a mass of flowable material and the void volume above it within a container may also further comprise the step of:
(iii) estimating the level of the interface between the mass of flowable material and the void volume above the mass of flowable material within the container using the level indicator where the estimated area falls between a level having a profound color change and a level having a faint color change.
These and other objects of the present invention are discussed in greater detail hereafter, in association with the accompanying drawings.